The present invention relates generally to turbomachines; and more particularly to a method and a system for independently limiting the steam flow entering a section of a steam turbine.
Steam turbines are commonly used in power plants, heat generation systems, marine propulsion systems, and other heat and power applications. Steam turbines typically include at least one section that operates within a pre-determined pressure range. This may include: a high-pressure (HP) section; and a reheat or intermediate pressure (IP) section. The rotating elements housed within these sections are commonly mounted on an axial shaft. Generally, control valves and intercept valves control steam flow through the HP and the IP sections, respectively.
The normal operation of a steam turbine includes three distinct phases; which are startup, loading, and shutdown. The startup phase may be considered the operational phase beginning in which the rotating elements begin to roll until steam is flowing through all sections. Generally, the startup phase does not end at a specific load. The loading phase may be considered the operational phase in which the quantity of steam entering the sections is increased until the output of the steam turbine is approximately a desired load; such as the rated load. The shutdown phase may be considered the operational phase in which the steam turbine load is reduced, and steam flow into each section is gradually stopped and the rotor, upon which the rotating elements are mounted, is slowed to a turning gear speed.
Some steam turbine operators employ a balanced flow strategy, during most of the loading phase. This strategy seeks to supply equal amounts of steam flow through each section. Here, a control system controls the steam flow via a command that positions the associated valves. Other control schemes are commonly used during the startup and shutdown operational phases.
During a startup of a steam turbine integrated with a cascade bypass system, steam may be diverted through a bypass valve to an intercept valve, while the control valve is substantially closed. Here, the intercept valve may perform the initial speed/load control of the steam turbine. Then, at a predetermined load range, the control valve primarily provides the speed/load control, while the intercept valve is biased open. Other operations may result in the significant loading of the IP section while steam flow into the HP section is considerably reduced. Consequently, the unbalanced flow may increase the net thrust on the rotor.
There are a few issues, with known methods of controlling the steam turbine during the startup, loading, and shutdown operational phases. Currently known methods may be disadvantageously conservative. These methods can reduce operational flexibility, require larger mechanical components, and potentially reduce the net-output delivered by the steam turbine. Therefore, there is a desire for a method and a system for increasing the operational flexibility of the steam turbine.